Turbine engines, such as single shaft industrial turbine engines, are designed to operate at a constant design turbine inlet temperature under any ambient air temperature (i.e., the compressor inlet temperature). This design turbine inlet temperature allows the engine to produce maximum possible power, known as base load. Any reduction from the maximum possible base load power is referred to as part load operation. In other words, part load entails all engine operation from 0% to 99.9% of base load power.
Part load operation may result in the production of high levels of carbon monoxide (CO) during combustion. One known method for reducing part load CO emissions is to bring the combustor exit temperature or the turbine inlet temperature near that of the base load design temperature. It should be noted that, for purposes of this disclosure, the terms combustor exit temperature and turbine inlet temperature are used interchangeably. In actuality, there can be from about 30 to about 80 degrees Fahrenheit difference between these two temperatures due to, among other things, cooling and leakage effects occurring at the transition/turbine junction. However, with respect to aspects of the present invention, this temperature difference is insubstantial.
To bring the combustor exit temperature closer to the base load design temperature, mass flow of air through the gas turbine can be restricted by closing the compressor inlet guide vanes (IGV), which act as a throttle at the inlet of the compressor. When the IGVs are closed, the trailing edges of the vanes rotate closer to the surface of an adjacent vane, thereby effectively reducing the available throat area. Reducing throat area reduces the flow of air which the first row of rotating blades can draw into the compressor. Lower flow to the compressor leads to a lower compressor pressure ratio being established by the turbine. As a result, less power can be extracted from the gas passing through the turbine, causing the turbine exhaust gases to become hotter at the turbine exit.
However, there is an exhaust temperature limit that the turbine components in the exhaust path, such as the exhaust manifold and diffuser, can withstand before degrading. For example, the exhaust temperature limit can be from about 1160 degrees Fahrenheit to about 1180 degrees Fahrenheit. Once the exhaust temperature limit is reached, combustion temperature must be dropped as load is further reduced. Thus, the goal of holding high combustor temperatures is thwarted by the exit temperature limit as load is reduced.